General Chemistry Principles, Patterns, and Applications, 2011

sodium halides increase from NaI to NaF. The fact that the solubilities decrease as the lattice energy
increases suggests that the ΔH2 term in dominates for this series of compounds.
A solution with the maximum possible amount of solute is saturated. If a solution contains less than the
maximum amount of solute, it is unsaturated. When a solution is saturated and excess solute is present,
the rate of dissolution is exactly equal to the rate of crystallization (part (b) in ). Using the value just
stated, a saturated aqueous solution of NaCl, for example, contains 35.9 g of NaCl per 100 mL of water at
20°C. We can prepare a homogeneous saturated solution by adding excess solute (in this case, greater
than 35.9 g of NaCl) to the solvent (water), stirring until the maximum possible amount of solute has
dissolved, and then removing undissolved solute by filtration.
Note the Pattern
The solubility of most solids increases with increasing temperature.
Because the solubility of most solids increases with increasing temperature, a saturated solution that was
prepared at a higher temperature usually contains more dissolved solute than it would contain at a lower
temperature. When the solution is cooled, it can therefore become supersaturated (part (c) in ). Like a
supercooled or superheated liquid (see ), a supersaturated solution is unstable. Consequently, adding a
small particle of the solute, a seed crystal, will usually cause the excess solute to rapidly precipitate or
crystallize, sometimes with spectacular results, as was shown in . The rate of crystallization in is greater
than the rate of dissolution, so crystals or a precipitate form (part (d) in ). In contrast, adding a seed
crystal to a saturated solution reestablishes the dynamic equilibrium, and the net quantity of dissolved
solute no longer changes.
Because crystallization is the reverse of dissolution, a substance that requires an input of heat to form a
solution (ΔHsoln > 0) releases that heat when it crystallizes from solution (ΔHcrys < 0). The amount of heat
released is proportional to the amount of solute that exceeds its solubility. Two substances that have a
positive enthalpy of solution are sodium thiosulfate (Na2S2O3) and sodium acetate (CH3CO2Na), both of
which are used in commercial hot packs, small bags of supersaturated solutions used to warm hands
(see ).
Interactions in Liquid Solutions
The interactions that determine the solubility of a substance in a liquid depend largely on the chemical
nature of the solute (such as whether it is ionic or molecular) rather than on its physical state (solid,
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